The DRAM (Dynamical Random Access Memory), since its appearance as a charge-storage memory, has been used widely as a semiconductor memory whose degree of integration has been increasing generation after generation. However degree of integration progresses for further reduction of element size, it is necessary the electric capacity of the capacitor to be maintained at about 30 fF. As a method for meeting this requirement, the study has been made on method for replacing the dielectrics film with a thin film in order to increase the effective area of the capacitor corresponding to the reduction in the size of the element.
On the other hand, it is considered that the lithographic method is cannot be only method capable of meeting the technical requirement for the giga-bit generation of semiconductors. For instance, there is an increasing demand for forming the wiring of the capacitor by using a new material in order to assure necessary performance of highly fined (wiring). The research for development and practical use of this kind and material are already under way.
It is considered effective for the giga-bit generation, for the reason described later, to use a material having a higher dielectric constant than those of SiO2 and Si3N4 such as so-called high dielectric material.
In order to prevent the reduction of the effective area of the capacitor resulting from the progress of the semiconductor for higher integration, the 3D-capacitor such as the trench cell and stack cell has been adopted. After the 1-giga-bit generation, the 3D-capacitor as is described above is becoming increasingly complex and fine, thereby suggesting the limitation of the manufacture of (further advanced) DRAM's. Further thinning of the capacitor insulation film invites the increase in leakage current caused by the tunnel current.
As high dielectric materials, the composite oxides such as SrTiO3, BaxSr1-x (BST) and the like are coming to the fore. Since these materials have the perovskite structure respectively, they give high dielectric constants respectively.
However, for example, in the case of the BST film formation by the CVD method, the problem such as the increase in the leakage current caused by the absence of Ti atom or the defective crystallization in the film resulting thinning of the film is becoming increasingly conspicuous.
On the other hand, the non-volatile high dielectric memory cell and the Ferroelectric RAM having its array, using the high dielectric film made of lead titanate perovskite compound or bismuth (layered) compound as the interelectrode insulation film of the capacitor for the storage of information, are coming to attract the attentions of those who concerned.
Being a non-volatile type, the Ferroelectric RAM has an advantage that not only it does not require refreshing operation for the holding of the data but also it does not require power consumption during stand-by period.
Further, the Ferroelectric RAM compared with the flash memory, which is also a kind of the non-volatile memory, has an advantage that data re-writing can be made a greater number of times and at a much higher speed.
In addition, the EEPROMs (Electrically Erasable Programmable Read Only Memories) which include flash or other memories have the following problems. The common problem of these memories is that they requires at least 3 power source voltages, thereby requiring a large power consumption. The storage of the information is done by injection of electron to and draw of electron from a floating gate through an insulation film called tunnel oxide. The injection and draw of electron causes the breakdown (fatigue) of the tunnel insulation film and the deterioration of the electric characteristic of the tunnel insulation film.
Further, compared with the power consumption of the SRAM (Static RAM), which can be backed up with a battery used for memory card and the like, the power consumption (of the Ferroelectric RAM) is smaller and has the potentiality for higher integration and further reduction in the area of the cell.
Having such a new function, the Ferroelectric RAM, as a next-generation memory, is considered to have an ability to replace the existing flash memory, SRAM and DRAM because of its potentiality for application to the logic device with embedded components and the like. Further, the Ferroelectric RAM is beginning to be used for the non-contact cards (FR-D: Radio Frequency-Identification), because of its advantage that it can operate at a high speed without battery.
However, the formation technique of the Ferroelectric RAM, as a highly integrated device, especially the technique for incorporating the ferroelectric film into the card, has not be established yet. For instance, if a ferroelectric film is formed by the sputtering method by using PbZrxTi1-xO3 (PZT), which is made of the ABO3 perovskite type structure, where the A means A-site atom, B means B-site atom, PZT film is apt to form the columnar crystal, especially on the ground electrode such as Pt electrode. Further, if the PZT film is affected by the roughness of the ground electrode, the surface morphology of the PZT film presents an extremely rough condition.
Further, when a pluralistic material such as SrRuO3(SRO), one of the conductive oxides, or the like is used as a constituent material of the ground electrode, this causes a problem that the constituent element of the SRO film diffuses in the PZT film with respect to the crystallization of the amorphous PZT film in the upper layer of the ground electrode (SRO film). This causes not only the deterioration of ferroelectric characteristic but also the increase in the amount of leakage current.
Further, in the case of the conventional method, in which the amorphous PZT film is crystallized starting from an interface of the lower (bottom) electrode, there occurs a problem that the Pb atoms diffuse in the lower electrode simultaneously with the crystallization, causing the defect of the Pb. Such problem can be alleviated by the method such as providing a buffer layer containing the (B-site) atoms such as Ti and supplying surplus amount of Pb in the vicinity of the lower electrode interface, but the omission of the Pb in the outermost surface lay cannot be prevented. Such Pb omission leads to the deterioration of the ferroelectric characteristic or the deterioration of the reliability such as the polarization fatigue characteristic.
Further, in consideration of the volatile matter of the Pb producing a high vapor pressure, the Pb content in the amorphous PZT film is increased by about several tens % (e.g., about 20%). Therefore, in the case where the conventional method, in which the amorphous PZT film is crystallized from the lower (bottom) interface, is employed, the surplus (excess) Pb, as is described above, segregates in the direction which the crystallization progresses. Consequently, the Pb tends to exists excessively on the surface region or in grain boundary of the PZT film, which has undergone crystallization process, and a layer having a low dielectric constant such as PbO tend to be formed on the above-described surface or in grain boundary. Such low-dielectric-constant layer forms the pass for the leakage current, causing large deterioration of high ferroelectric characteristic.
Further, in the interface between the lower or upper capacitor electrode and upper capacitor electrode, asymmetry of the PZT film occurs in the direction of the film thickness. Or, the defect of the crystallization can tend to concentrate in the vicinity of the interface of one side because of that the crystallization starts only from one direction, that is, from the interface of the lower electrode. Such factor leads to the deterioration of the polarization fatigue or degradation of switching endurance characteristic. Further, when such capacitor is applied to the memory, this causes the occurrence of the problem such as the decline of the reliability concerning the operation of the memory, such as the occurrence of imprint phenomenon and poor retention characteristic.